The Sun and the rhythm of light

Life on Earth evolved under the Sun’s predictable cycles. For millions of years, our cells learned to read the subtle shifts in the wavelengths of sunlight across the day. Today, the science of photobiology and chronobiology reveals that natural light does more than let us see: it acts as a scheduled signal that actively primes, protects, and repairs the body.

Understanding how the different light spectra interact —from the morning infrared to the midday ultraviolet— is essential to optimise our health, synthesise essential nutrients safely, and prevent premature ageing.

1. Dawn: the “light shield” and subcellular melatonin

The day does not begin with harsh light. In the early hours of the morning, because of the Sun’s angle, its rays travel through a thicker layer of the atmosphere. This phenomenon (known as Rayleigh scattering) filters out high-energy light (blue and ultraviolet) and lets red light predominate along with, invisibly, near-infrared light (NIR — Near Infrared).

The biological mechanism: photo-preconditioning

Unlike conventional visible light, near-infrared has a unique ability to penetrate deep into the body’s tissues, passing through the skin and even reaching the underlying organs. As it interacts with cells, it triggers a critical process:

• Mitochondrial activation: infrared is absorbed by an enzyme inside our mitochondria called cytochrome c oxidase.
• Subcellular melatonin: this stimulation drives the large-scale production of mitochondrial melatonin. Unlike the melatonin produced by the pineal gland at night (which travels through the blood and induces sleep), mitochondrial melatonin is generated locally inside each cell.
• The antioxidant “vaccine”: this melatonin acts as the most potent intracellular antioxidant known. Its role at this time of the morning is to prepare the ground. By raising the body’s antioxidant defences (enzymes such as catalase and superoxide dismutase), the light of dawn performs a photo-preconditioning. It readies and “vaccinates” the cells to withstand the oxidative stress and destructive radiation damage that will arrive a few hours later.

2. Noon: the spectrum of synthesis and energy

As the Sun reaches its highest point in the sky (the central hours of the day), the angle of its rays becomes perpendicular to the atmosphere. This reduces the atmospheric filter and allows two high-energy spectra to reach the Earth’s surface: blue light and ultraviolet radiation.

The blue-light myth and the true source of vitamin D

There is a common confusion in which midday blue light is credited with producing nutrients in the skin. Physics and biology, however, divide the roles precisely:

• Blue light (visible spectrum): it is essential for the eyes. When it strikes the retina’s ganglion cells, it halts night-time melatonin production and activates cortisol, telling the brain it is time to be alert, focused, and full of energy. In the skin, though, blue light produces no vitamins; in excess it generates free radicals and pigmentation.
• Ultraviolet B radiation (UVB): the engine of vitamin D. Vitamin D synthesis is a process strictly dependent on UVB radiation (wavelengths between 280 and 315 nanometres). When UVB photons hit the epidermis, they break the bonds of a molecule called 7-dehydrocholesterol, turning it into previtamin D3, which is then converted into active vitamin D via the liver and kidneys.

Thanks to the photo-preconditioning received from the infrared light of dawn, the skin cells have the antioxidant resources they need to mitigate the DNA damage and burns that UV rays can cause during this synthesis process.

3. Dusk: the shift for repair and regeneration

Toward the end of the day, the Sun tilts back down to the horizon. Ultraviolet radiation disappears entirely and the atmosphere once again bathes the surroundings in an absolute predominance of red and near-infrared light. Physically, the light of dusk has the same wavelength as that of dawn.

Its biological impact, however, is radically opposite, because of the state of the body receiving it.

Same light, different cellular context

The difference in effect lies in the circadian rhythm and in the tissue’s history over the previous 12 hours:

1. Clearing the day’s damage: by the end of the day, skin cells have already accumulated micro-injuries, inflammation, and free radicals from midday sun exposure, pollution, and metabolic stress.
2. Active repair mechanism: when the infrared of dusk penetrates this damaged tissue, mitochondrial stimulation no longer acts as prevention but as a repair switch. It activates cell-signalling pathways (such as the p53 protein) aimed at repairing damaged DNA strands, sharply reduces pro-inflammatory enzymes (such as COX-2), and stimulates collagen production to heal the skin’s matrix.
3. Transition to rest: in parallel, the disappearance of blue light from the ambient spectrum sends the biological signal to the brain that night has come, allowing pineal melatonin secretion to begin the restorative sleep cycle.

4. The great modern mismatch: lifestyle versus ancestral biology

To understand the current role of sunscreens, we must set out a fundamental biological premise: the Sun is neither a mistake of nature nor an intrinsic enemy of our health; the real mistake is the modern lifestyle. Human beings evolved over hundreds of thousands of years keeping a daily, direct, and symbiotic relationship with sunlight —a bond that industrialised society has broken abruptly in recent decades.

The ancestral shield versus the “office-beach” effect

Under ancestral living conditions, or with continuous outdoor exposure, chemical or artificial sunscreens are biologically dispensable. This is because the human body has a perfectly coordinated, progressive defence system:

• Seasonal acclimatisation: living exposed to the environment, our ancestors experienced a gradual increase in radiation from winter to summer. This slow rise let the skin thicken its corneal layer and build up melanin sustainably, creating a natural, biological “sunscreen”.
• Daily photo-preconditioning: the obligatory exposure to the early hours of dawn charged the mitochondria with subcellular melatonin, preparing tissues for the impact of the midday UVB rays.

Modern humans, by contrast, suffer the so-called “office-beach effect”. From Monday to Friday we are confined indoors, chronically deprived of morning red light and with skin unaccustomed to and lacking protective melanin. When the weekend or holidays arrive, we expose the body abruptly and massively to extreme doses of UV radiation in the central hours of the day. In this scenario of circadian and biological mismatch, natural defences are completely overwhelmed, making sunscreen a mandatory containment tool to prevent acute cellular damage.

The paradox of sunscreen toxicity

The massive, indiscriminate use of sunscreens as a substitute for sensible sun exposure has opened a new public-health debate because of the makeup of their ingredients. Many conventional chemical-based filters (organic filters) do not stay on the skin’s surface but are absorbed systemically into the bloodstream, presenting documented risks:

• Endocrine disruption: widely used compounds such as oxybenzone (benzophenone-3), octinoxate, and homosalate have been shown in several studies to act as hormonal disruptors. These molecules mimic or interfere with the body’s natural hormones (especially oestrogens and thyroid hormones), altering the endocrine system.
• Carcinogenic potential and contaminants: the debate over the safety of these products intensified after findings of benzene contamination —a known human carcinogen— in numerous batches of commercial spray sunscreens. Likewise, some chemical filters can, under UV radiation, degrade and paradoxically increase free radicals in the skin itself if they are not properly formulated.
• Environmental impact: these substances do not only affect human biochemistry; dissolved in water, they cause the bleaching and death of coral reefs, gravely disturbing marine ecosystems.

Sunscreen should be understood as an emergency mechanism against the shortcomings of modern life, not as a free pass to artificially prolong exposure to the midday sun. To minimise the toxicological impact, however, the scientific recommendation is to prioritise pure physical or mineral filters (such as non-nano zinc oxide), which act as an inert barrier on the skin without being absorbed or altering the endocrine system, restoring logic and safety to our relationship with the Sun.

5. Conclusion

The human body is not a static system; it is an organism rhythmically coordinated by the physics of the sky. Designing an intelligent sun-exposure routine means understanding this biological order:

• Seek the dawn to build internal cellular resilience.
• Use the noon with brief, controlled exposures to secure optimal vitamin D levels through UVB radiation.
• Embrace the dusk to speed tissue recovery and prepare the body for night-time rest.

Breaking this balance —for example, spending the day under artificial office light and then exposing yourself to the sun only abruptly at noon— deprives cells of their natural defence and repair mechanisms, increasing the risk of chronic skin damage.

6. References and scientific support

To explore the clinical and molecular evidence behind this article, the following scientific literature is suggested:

• Zimmerman, S., & Reiter, R. J. Melatonin: Both a Messenger of Darkness and a Participant in the Cellular Actions of Non-Visible Solar Radiation of Near Infrared Light. Melatonin Research. Link
• Beyond Sleeping Disorders, the Role of Melatonin in Skin Diseases and Emerging Applications in Dermatology and Topical Therapy. MDPI / PubMed Central. Link
• Vitamin D Metabolism, Mechanism of Action, and Clinical Applications. Journal of Advanced Research / NIH. Link
• Unlocking the Power of Light on the Skin: A Comprehensive Review on Photobiomodulation. PubMed Central. Link